Vehicle traveling support apparatus

ABSTRACT

A driving support ECU is configured to, when a specific recognition state occurs where a lane marker recognized changes from a left lane marker to a right lane marker or vice versa under a one side lane marker recognizable state, set a steering angle guard value to a second steering angle guard value smaller than a first steering angle guard value and set a steering angle speed guard value to a second steering angle speed guard value smaller than a first steering angle speed guard value.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a vehicle traveling support apparatusfor performing a lane keeping support/assist control which supports atravel of a vehicle (an own vehicle) by using white lines in such amanner that the own vehicle travels/runs in the vicinity of the centerof a lane.

2. Description of the Related Art

A vehicle traveling support apparatus, which has been conventionallyknown, recognizes lane markers (hereinafter, also simply referred to as“white lines”) such as a white fine, a yellow line, and the like on aroad by using a lane marker recognizing device (for example, at leastany one of a camera sensor and a radar sensor) provided on an ownvehicle to perform a lane keeping support control which supports atravel of a vehicle (an own vehicle) by using the recognized white linesin such a manner that the own vehicle travels in the vicinity of thecenter of a lane.

For example, one example of the vehicle traveling support apparatusesdetects the left and right white lines on a traveling lane in which theown vehicle is traveling using the camera sensor and sets/determines, asa target traveling line, a predetermined position (for example, thecenter position in the left and right white lines) based on positions ofthe detected left and right white lines.

Then, the vehicle traveling support apparatus controls a steering angleof the own vehicle so as to have the own vehicle travel along the settarget traveling line.

Furthermore, as a lane marker recognizing technique which can be appliedto such a vehicle traveling support apparatus, in a situation where oneof the left and right white lines is unable to be recognized, a devicehas been known, which extrapolates a position of the unrecognized whiteline by using the recognized white line and an extrapolated lane width(for example, refer to Japanese Patent Application Laid-Open No.2002-163642).

The inventor of the present application has studied the vehicletraveling support apparatus which uses, as the target traveling line, aline passing through positions, each of which is determined based on onewhite line which has been recognized and the extrapolated/estimated lanewidth, when one of the left and right white lines cannot be recognized.

However, in this apparatus, the following problems may occur, That is,when a specific recognition state occurs where the one recognizablewhite line changes from the left white line (the left lane marker) tothe right white line (the right lane marker) or vice versa, a positionof the target traveling line may change greatly from a position at atime point immediately before the specific recognition state occurs.When the position of the target traveling line changes greatly, asteering angle of the own vehicle changes greatly. As a result, abehavior of the own vehicle becomes unstable. It should be noted that areason why the position of the target traveling line changes greatlybefore and after the specific recognition state occurs will be describedlater.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve theabove-described problem. That is, one of objects of the presentinvention is to provide a vehicle traveling support apparatus which canreduce a possibility that a traveling stability of an own vehicledegrades due to a great change in steering the own vehicle, when theabove-described specific recognition state occurs while one of left andright white lines is being unable to be recognized. Hereinafter, thevehicle traveling support apparatus according to the present inventionmay be referred to as a “present invention apparatus”.

The present invention apparatus includes:

lane marker recognition means (10, 17) for recognizing a lane marker ona traveling lane in which an own vehicle is traveling;

control means (10) for performing a lane keeping control to control asteering angle of the own vehicle so as to have the own vehicle travelalong a target traveling line (Ld) set based on at least the lanemarker; and

estimating lane width means (10) for estimating a lane width of thetraveling lane.

The control means is configured:

-   -   when a recognition state of the lane marker by the lane marker        recognition means is a both lane markers recognizable state        which is a state where the lane marker recognition means can        recognize both a left lane marker (LL) on a left side with        respect to the own vehicle and a right lane marker (LR) on a        right side with respect to the own vehicle (refer to a “No”        determination at step 525),

to set the target traveling line based on the left lane marker and theright lane marker (refer to step 526); and

to perform a first limiting process (refer to step 528) including oneof,

-   -   a first steering angle magnitude limiting process for limiting        the steering angle in such a manner that a magnitude of the        steering angle does not exceed a first steering angle guard        value, and    -   a first steering angle speed magnitude limiting process for        limiting the steering angle in such a manner that a magnitude of        a steering angle speed which is an amount of change per unit        time of the steering angle does not exceed a first steering        angle speed guard value.

It should be noted that the estimating lane width means may acquire adistance between the left lane marker and the right lane marker as thelane width (W) in the both lane markers recognizable state to use theacquired lane width (W) as a lane width (W1), when the recognition stateof the lane marker is a one side lane marker recognizable state whichwill be described later, thereby estimating the lane width (W1).Furthermore, the estimating lane width means may acquire the lane width(W) from an external network system by a communication with the externalnetwork system, thereby estimating the lane width (W1) when therecognition state of the lane marker is the one side lane markerrecognizable state which will be described later.

As shown in FIG. 4, the recognition state of the lane marker may changefrom the both lane markers recognizable state (a time t0 to a time 1) tothe one side lane marker recognizable state where only one of the whitelines (lane markers) (the time t1 to a time t3) can be recognized. Inthe one side lane marker recognizable state, the above control meanscannot set the target traveling line based on both the left and rightlane markers.

In view of the above, the control means is configured:

to set the target traveling line (refer step 535) based on the lanemarker which is recognized by the lane marker recognition means and theestimated lane width (W1), when the recognition state of the lane markerby the lane marker recognition means is the one side lane markerrecognizable state which is a state where the lane marker recognitionmeans can recognize only one of the left lane marker and the right lanemarker (refer to a “Yes” determination at step 525).

However, as shown in FIG. 4, while the recognition state of the lanemarker is the one side lane marker recognizable state (the time t1 tothe time t3), a case may occur where the estimated lane width (W1) andthe true lane width (Wtrue) are greatly different from each other. Inthis case, when the left white line continues to be recognized beforeand after the time t1, (a position of) the target traveling line doesnot change greatly (refer to Ld1 and Ld2).

However, immediately after the time t2 at which the recognizable lanemarker changes from the left white line to the right white line(immediately after a time point at which the above-described specificrecognition state occurs), setting the target traveling line (Ld3) basedon the position of the right white line and the lane width (W1=W) causesa great change in the position of the target traveling line (refer toLd2 and Ld3). As a result, a deviation/difference between the positionof the own vehicle and the target traveling line becomesabruptly/suddenly large. Therefore, the steering angle is changedgreatly by the lane keeping control, so that a traveling stability ofthe own vehicle is likely to degrade.

In view of the above, the control means is configured

to perform, when a specific recognition state occurs where the lanemarker recognized by the lane marker recognition means changes from theleft lane marker to the right lane marker or vice versa (refer to a“Yes” determination at step 615 and step 625) under the one side lanemarker recognizable state (refer to a “Yes” determination at step 525),a second limiting process (refer to step 550) including one of,

-   -   a second steering angle magnitude limiting process, in place of        the first steering angle magnitude limiting process, for        limiting the steering angle in such a manner that the magnitude        of the steering angle does not exceed a second steering angle        guard value smaller than the first steering angle guard value,        and    -   a second steering angle speed magnitude limiting process, in        place of the first steering angle speed magnitude limiting        process, for limiting the steering angle in such a manner that        the magnitude of the steering angle does not exceed a second        steering angle speed guard value smaller than the first steering        angle speed guard value.

This allows any one of the maximum magnitude of the steering angle andthe maximum magnitude of the steering angle speed to become smaller, ascompared with when the recognition state of the lane marker is the bothlane markers recognizable state. As a result, a yaw rate of the ownvehicle does not change abruptly/suddenly, and thus, the possibilitythat the traveling stability of the own vehicle degrade can be reduced.

In one of aspects of the present invention apparatus,

the control means is configured to perform the first limiting process(refer to step 528) in place of the second limiting process when thespecific recognition state has occurred (refer to a “No” determinationat step 630 and step 635) after the own vehicle crossed the lane markerrecognized by the lane marker recognition means, while the recognitionstate of the lane marker by the lane markers recognition means is theone &de lane marker recognizable state.

The above-described specific recognition state may also occurimmediately after the own vehicle crosses the one white line that hasbeen recognized after the position in the vehicle width direction of theown vehicle changes so as to approach the white line for some reason. Insuch a case, the target traveling line is rarely changed. Therefore,even if the second guard limiting process is not performed, the steeringstate of the own vehicle is unlikely to change abruptly/suddenly.Conversely, it is rather preferable to cause the own vehicle to quicklyapproach the target traveling line through setting the guard valuesrelating the steering angle (that is, the steering angle guard value andthe steering angle speed guard value) to large values using the firstlimiting process. Furthermore, in such a situation, if the targettraveling line is changed to “a target traveling line for a changed laneon the side to which the own vehicle has moved”, the own vehicle shouldbe made to come closer to the changed target traveling line morequickly. For this reason, it is not necessary to perform the secondlimiting process. Therefore, according to the above-mentioned aspect,when the second limiting process is unnecessary, the second limitingprocess is not performed, so that the lane keeping control can beappropriately performed.

In one of aspects of the present invention apparatus,

the control means is configured, when a particular state has continuedfor a predetermine period from a time point at which the specificrecognition state occurred (refer to a “Yes” determination at step 635and step 640), the particular state being a state where the recognitionstate of the lane marker by the lane marker recognition means is the oneside lane marker recognizable state and the particular recognition statedoes not occur, to perform a third limiting process including at leastany one of,

-   -   a third steering angle magnitude limiting process, in place of        the second steering angle magnitude limiting process, for the        steering angle in such a manner that the magnitude of the        steering angle does not exceed a third steering angle guard        value larger than the second steering angle guard value and        equal to or smaller than the first steering angle guard value;        and    -   a third steering angle speed magnitude limiting process, in        place of the third steering angle speed magnitude limiting        process, for limiting the steering angle speed in such a manner        that the magnitude of the steering angle speed does not exceed a        third steering angle speed guard value larger than the second        steering angle speed guard value and equal to or smaller than        the first steering angle speed guard value.

When the above particular state is continued for the above predeterminedperiod from the time point at which the specific recognition state hasoccurred, a sufficient time has elapsed after the target traveling linechanged greatly, and thus, the own vehicle is likely to travel in thevicinity of the target traveling line. Therefore, after this time point,even if the guard values relating the steering angle (that is, thesteering angle guard value and the steering angle speed guard value) areset to the large values, the traveling stability of the own vehicle isunlikely to degrade. Therefore, according to the above-mentioned aspect,when the second limiting process is unnecessary, the third limitingprocess is performed, so that the lane keeping control can beappropriately performed.

It should be noted that the third steering angle guard value used in theabove third steering angle magnitude limiting process may be the same asthe above first steering angle guard value. Note that, however, sincereliability of the target traveling line under the one lane markerrecognizable state is lower than reliability of the target travelingline under the both lane markers recognizable state, it is preferablethat “the third steering angle guard value used in the above thirdsteering angle magnitude limiting process be larger than the abovesecond steering angle guard value and be smaller than the above firststeering angle guard value”.

Similarly, the third steering angle speed guard value used in the abovethird steering angle speed magnitude limiting process may be the same asthe first steering angle speed guard value. It should be noted, however,as mentioned above, that the reliability of the target traveling lineunder the one lane marker recognizable state is lower than thereliability of the target traveling line under the both lane markersrecognizable state. Thus, it is preferable that “the third steeringangle speed guard value used in the above third steering angle speedmagnitude limiting process be larger than the above second steeringangle speed guard value and be smaller than the above first steeringangle speed guard value.”

In one of aspects of the present invention apparatus,

the control means is configured to determine that the particular statehas continued for the predetermine period, when a duration time of theparticular state from a time point at which the specific recognitionstate occurred becomes equal to or longer than a threshold durationtime.

In one of aspects of the present invention apparatus,

the control means is configured to determine that the particular statehas continued for the predetermine period, when a traveling distance ofthe own vehicle under the particular state from a time point at whichthe specific recognition state occurred becomes equal to or longer thana threshold traveling distance.

In the above description, references used in the following descriptionsregarding embodiments are added with parentheses to the elements of thepresent invention, in order to assist in understanding the presentinvention. However, those references should not be used to limit thescope of the invention. Other objects, other features, and accompanyingadvantages of the present invention will be readily understood from thedescription of embodiments of the present invention to be givenreferring to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a vehicle travelingcontrol apparatus according to an embodiment of the present invention.

FIG. 2 is a plan view for illustrating a left white line, a right whiteline, a target traveling line, and a curve radius.

FIG. 3 is a plan view for illustrating a road and a vehicle forexplaining a lane keeping control.

FIG. 4 is a plan view for illustrating the road and the vehicle forexplaining an operation of the vehicle traveling control apparatus.

FIG. 5 is a flowchart for illustrating a routine executed by a CPU of adriving support ECU shown in FIG. 1.

FIG. 6 is a flowchart for illustrating a routine executed by the CPU ofthe driving support ECU shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A vehicle traveling control apparatus (vehicle traveling support/assistapparatus) according to an embodiment of the present invention will bedescribed below, referring to the drawings.

<Construction>

As shown in FIG. 1, the vehicle traveling control apparatus(hereinafter, referred to as an “embodiment apparatus”) according to theembodiment of the present invention is applied to a vehicle. The vehiclewill be referred to as an “own vehicle” in order to distinguish thevehicle, to which the embodiment apparatus is applied, from the othervehicles. The embodiment apparatus includes a driving support ECU 10, anengine ECU 30, a brake ECU 40, a steering ECU 60, a meter ECU 70, analert ECU 80, a navigation ECU 90, and an external communication ECU100. It should be noted that the driving support ECU 10 is simplyreferred to as a “DSECU”, in the following description.

Each of those ECUs is an electronic control unit comprising amicrocomputer as a main part. The ECUs are connected with each other viaCAN (Controller Area Network) which is not illustrated so that they arecapable of mutually transmitting and receiving information. In thepresent specification, the microcomputer includes a CPU, a ROM, a RAM, anon-volatile memory, an interface I/F, and the like. The CPU isconfigured to realize/achieve various functions through executinginstructions (programs, routines) stored in the ROM. Some or all ofthose ECUs may be integrated into a single ECU.

The DSECU is connected to sensors (including switches) described belowand is configured to receive a detection signal or an output signal ofeach of the sensors. It should be noted that each of the sensors may beconnected to one of the ECUs other than the DSECU. In this case, theDSECU receives the detection signal or the output signal of the sensorvia CAN from the ECU to which that sensor is connected.

An accelerator pedal operation amount sensor 11 is configured to detectan operation amount (an accelerator opening degree) of an acceleratorpedal 11 a of the own vehicle to output a signal indicative of thedetected operation amount (the accelerator pedal operation amount AP). Abrake pedal operation amount sensor 12 is configured to detect anoperation amount of a brake pedal 12 a to output a signal indicative ofthe detected operation amount (the brake pedal operation amount BP).

A steering angle sensor 14 is configured to detect a steering angle ofthe own vehicle to output a signal indicative of the detected steeringangle (the steering angle θ). It should be noted that the steering angleθ is defined to be a positive value when a steering wheel SW is rotatedin the left direction from the neutral position and the steering angle θis defined to be a negative value when the steering wheel SW is rotatedin the right direction from the neutral position. A steering torquesensor 15 is configured to detect a steering torque applied to asteering shaft US of the own vehicle through an operation of thesteering wheel SW to output a signal indicative of the detected steeringtorque (the steering torque Tra). A vehicle speed sensor 16 isconfigured to detect a traveling speed (a vehicle speed) of the ownvehicle to output a signal indicative of the detected vehicle speed (thevehicle speed SPD). It should be noted that the DSECU is configured tocalculate a steering angle speed (=dθ/dt) which is an amount of changeper unit time of the steering angle θ received from the steering anglesensor.

The camera sensor 17 includes a stereo camera and a processing part tophotograph (take an image of) views/landscapes of the left and the rightside area in front (ahead) of the own vehicle so as to acquire images ofthe views/landscapes at the right and left sides of the own vehicle,every time a predetermined time elapses. The camera sensor 17 transmitsthe photographed images to the DSECU.

The DSECU executes an image processing for the image data received froma camera sensor 17 to acquire an image processing data. The DSECUacquires (recognizes) lane markers (hereinafter, simply referred to as“white lines”) on the road in which the own vehicle is traveling byusing the image processing data. The DSECU recognizes, as a left whiteline, a white line on the left side of the own vehicle in the travelingdirection of the own vehicle. The DSECU recognizes, as a right whiteline, a white line on the right side of the own vehicle in the travelingdirection of the own vehicle.

An operation switch 18 is a switch operated by a driver. The driver canoperate the operation switch 18 to select whether or not a lane keepingcontrol is to be performed. It should be noted that the lane keepingcontrol is also referred to as “LKA (a lane keeping assist control)”.

A yaw rate sensor 19 is configured to detect a yaw rate of the ownvehicle to output a signal indicative of the detected yaw rate (anactual yaw late YRt).

The engine ECU 30 is connected to an engine actuator 31. The engineactuator 31 is an actuator for changing an operation state of aninternal combustion engine 32. In the present example, the internalcombustion engine 32 is a gasoline fuel injection·spark ignitiontype·multi-cylinder engine, and includes a throttle valve for adjustingan intake air amount of the internal combustion engine 32. The engineactuator 31 includes at least a throttle valve actuator for changing anopening degree of the throttle valve. The engine ECU 30 changes a torquegenerated by the internal combustion engine 32 through driving theengine actuator 31. The torque generated by the internal combustionengine 32 is transmitted to driving wheels (not shown) via atransmission (not shown). Therefore, the engine ECU 30 can control theengine actuator 31 to control a driving force of the own vehicle, tothereby be able to change an acceleration state (acceleration) of theown vehicle.

The brake ECU 40 is connected to a brake actuator 41. The brake actuator41 is provided in a hydraulic pressure circuit provided between a mastercylinder 45 (not shown) for pressurizing hydraulic oil using adepression force of the brake pedal 12 a and a friction brake mechanism42 provided in right-and-left front-and-rear wheels. The friction brakemechanism 42 includes brake discs 42 a each secured to the correspondingwheel of the own vehicle and brake calipers 42 b each secured to thebody of the own vehicle at the corresponding wheel. The brake actuator41 adjusts, in response to an instruction from the brake ECU 40, ahydraulic pressure to be supplied to a wheel cylinder built in the brakecaliper 42 b of the friction brake mechanism 42 to press a brake padonto a brake disk 42 a using the hydraulic pressure so as to generate afriction braking force. Therefore, the brake ECU 40 can control thebrake actuator 41 to control a braking force applied to the own vehicleSV.

The steering ECU 60 is a control unit of a well-known electric powersteering system and is connected to a motor driver 61. The motor driver61 is connected to a steering motor 62. The steering motor 62 isincorporated into a “steering mechanism including a steering wheel, asteering shaft coupled to the steering wheel, a gear mechanism forsteering, and the like” of the vehicle. The steering motor 62 generatestorque using electric power supplied from the motor driver 61, tothereby be able to add a steering assist torque to the steeringmechanism or steer the left-and-right steered wheels using the generatedtorque. That is, the steering motor 62 can change the steering angle ofthe own vehicle.

The meter ECU 70 is connected to a digital display meter (not shown), ahazard lamp 71, and a stop lamp 72. The meter ECU 70 blinks the hazardlamp 71 and lights the stop lamp 72, in response to instructionstransmitted from the DSECU.

The alert ECU 80 is connected to a buzzer 81 and a display device 82.The alert ECU 80 can have the buzzer 81 generate sounds to alert thedriver in response to an instruction transmitted from the DSECU. Inaddition, the alert ECU 80 can cause the display device 82 to light anattention/alert mark such as a warning lamp and/or to display anattention/alert message and an operation state of a driving supportcontrol. It should be noted that the display device 82 is a displaydevice for displaying an image in response to an instruction transmittedfrom the DSECU. Specifically, the display device 82 is a head-updisplay. The display device 82 may be another type of display such as amultifunction display.

The navigation ECU 90 is connected to a GPS receiver 91 which receives aOPS detection signal for detecting a present position of the ownvehicle, a map database 92 which stores a map information and the like,a touch-screen type display 93 which is a human-machine interface, andthe like. The navigation ECU 90 identifies the present position of theown vehicle based on the GPS detection signal. When the own vehicle istraveling in one of a plurality of lanes on a road, the navigation ECU90 can acquire information for identifying the lane in which the ownvehicle is traveling currently. The navigation ECU 90 executes variouscalculations based on the present position of the own vehicle, the mapinformation stored in the map database 92 and the like to perform aroute guidance using the display 93.

The map information stored in the map database 92 includes roadinformation. For example, the road information includes a lane width,and a road curvature radius (or a road curvature) which represents adegree of a curve of the road.

The external communication ECU 100 is connected to a wirelesscommunication device 101. The external communication ECU 100 and thewireless communication device 101 constitutes a wireless communicationterminal for communicating with an external network system. The wirelesscommunication device 101 is also capable of performing a wirelesscommunication with a roadside communication device installed on a road.The wireless communication includes both an optical wirelesscommunication and a wireless communication using an electric wave.

<Outline of Operation>

An outline of operation performed by the embodiment apparatus will nextbe described. The DSECU of the embodiment apparatus is capable ofperforming the lane keeping control. Hereinafter, the lane keepingcontrol will be described.

<<Lane Keeping Control>>

The DSECU performs the lane keeping control which will be describedlater, while the execution of the lane keeping control is requestedthrough the operation of the operation switch 18.

The DSECU recognizes (detects) the left and right white lines on theroad through analyzing the image data transmitted from the camera sensor17. The DSECU recognizes the white line on the left side with respect tothe center axis of the own vehicle as a left white line LL and the whiteline on the right side with respect to the center axis of the ownvehicle as a right white line LL. The DSECU recognizes an area betweenthe left white line LL and the right white line LR as a traveling lane(a lane) in which the own vehicle SV is traveling. The DSECUsets/determines a target traveling line Ld within the lane using thedetected white lines. That is, in this embodiment, the DSECUsets/determines a “center line which is positioned at the center betweenthe left white line LL and the right white line LR” to be (or as) the“target traveling line Ld”.

The DSECU performs a steering control for applying the steering torqueto the steering mechanism to change the steering angle (turning angle)of the own vehicle SV in such a manner that a lateral position (that is,a position of the own vehicle SV with respect to the traveling lane in avehicle width direction) of the own vehicle SV is kept in the vicinityof the target traveling line Ld within the traveling lane.

Meanwhile, for example, a case may occur where the camera sensor 17cannot detect (recognize) one or both of the white lines due to a roadsurface condition or deterioration of the white lines. Thus, the DSECUmay not always be able to recognize both of the left white line LL andthe right white line LR. That is, a state (referred to as a “recognitionstate of the white lines”) on how the DSECU recognizes the white linesis any one of the followings.

-   -   A state where both the left white line LL and the right white        line LR can be recognized. Hereinafter, this state is also        referred to as a “both white lines recognizable state” or a        “both lane markers recognizable state”.    -   A state where one of the left white line LL and the right white        line LR can be recognized, but the other white line cannot be        recognized. In other words, this is a state where only one of        the left white line LL and the right white line LR can be        recognized. Hereinafter, this state is also referred to as a        “one side white line recognizable state” or a “one side lane        marker recognizable state”.    -   A state where the DSECU cannot recognize any of the left white        line LL and the right white line LR. Hereinafter, this state is        also referred to as a “no white line recognizable state” or a        “no lane marker recognizable state”.

In view of the above, as will be described later, the DSECUappropriately changes a setting method of the target traveling linedepending on (in response to) each of the recognition state of the whitelines to perform the steering control for the lane keeping control.

<<A Case where Both of the Left White Line LL and the Right White LineLR can be Recognized (the Both White Lines Recognizable State)>>

As shown in FIG. 2, when the DSECU can recognize both of the left whiteline LL and the right white line LR, the DSECU sets/determines, as thetarget traveling line Ld, the lane center line positioned at a midpointbetween the recognized left white line LL and the recognized right whiteline LR.

In addition, the DSECU calculates a curve radius R of the targettraveling line Ld, a curvature CL (=1/R) of the target traveling lineLd, a position of the own vehicle SV in the traveling lane defined bythe left white line LL and the right white line LR, and a direction ofthe own vehicle SV in the traveling lane.

More specifically, as shown in FIG. 3, the DSECU calculates a distancedL (a center distance dL) in a road width direction between the centerposition in the front end of the own vehicle and the target travelingline Ld, and a deviation angle θL (a yaw angle θL) formed between adirection of the target traveling line Ld and a traveling direction ofthe own vehicle SV.

The DSECU calculates a target yaw rate YRc* based on the center distancedL, the yaw angle θL, and the curvature CL by using the followingExpression (1), every time a predetermined time elapses. In Expression(1), each of K1, K2, and K3 is indicative of control gain. The targetyaw rate YRc* is indicative of a yaw rate which is set in such a mannerthat the own vehicle SV can travel along the target traveling line Ld.YRc*=K1×dL+K2×θL+K3×CL  (1)

The DSECU calculates a target steering torque Tr* forrealizing/accomplishing the target yaw rate YRc* based on the target yawrate YRc* and the actual yaw rate YRt, every time the predetermined timeelapses. More specifically, the DSECU previously stores a look-up tablewhich defines a relationship between the target steering torque Tr* anda difference/deviation between the target yaw rate YRc* and the actualyaw rate YRt. The DSECU calculates/obtains the target steering torqueTr* by applying the difference/deviation between the target yaw rateYRc* and the actual yaw rate YRt to the look-up table.

The DSECU controls the steering motor 62 by using the steering ECU 60 insuch a manner that the actual steering torque Tra corresponds (becomesequal) to the target steering torque Tr*. In this manner, the DSECUperforms the lane keeping control which controls the steering angle ofthe own vehicle SV so as to have the own vehicle SV travel along thetarget traveling line Ld. It should be noted that the DSECUacquires/obtains a lane width W (that is, a distance between the leftwhite line LL and the right white line LR) based on positions of therecognized left white lines LL, LR, and stores the acquired lane width Win the RAM, every time a predetermined time elapses.

The DSECU may perform the lane keeping control as follows. That is, theDSECU sets a target value of the center distance dL to “0” and sets atarget value of the yaw angle θL to “0”. The DSECU calculates the targetsteering angle θ* by applying a difference/deviation (that is, theactual center distance dL) between the actual center distance dL and itstarget value, a difference/deviation (that is, the actual yaw angle θL)between the yaw angle θL and its target value, and the curvature CL tothe following Expression (2). Furthermore, the DSECU controls thesteering motor 62 by using the steering ECU 60 in such a manner that theactual steering angle θ corresponds (becomes equal) to the targetsteering angle θ*. In Expression (2), each of Klta1, Klta2, and Klta3 isindicative of control gain.θlta*=Klta1·CL+Klta2·θL+Klta3·dL  (2)

It should be noted that the lane keeping control is also referred to asLTA (Lane Tracing Assist). The basic control content of the LTA itselfis well known (for example, refer to Japanese Patent ApplicationLaid-Open No. 2008-195402, No. 2009-190464, No. 2010-6279, JapanesePatent No. 4349210 and the like).

<<A Case where the DSECU can Recognize Only One of the Left White LineLL and the Right White Line LR and Cannot Recognize the Other (the OneSide White Line Recognizable State)>>

A: When the DSECU can Recognize Only the Left White Line.

When the DSECU can recognize only the left white line LL on thetraveling lane in which the own vehicle is traveling, the DSECUextrapolates/determines, as a position of the right white line LR, aposition which is obtained through shifting a position of the recognizedleft white line LL to the right side in the lane width direction by thedistance of the estimated/inferred lane width W1. The estimated/inferredlane width W1 is a latest value of the lane width W which was calculatedwhen both of the left white line LL and the right white line LR wererecognized. The latest value of the lane width has been stored in theRAM. The DSECU determines, as the target traveling line Ld, a linepassing through positions, each of which is a position obtained throughshifting the recognized left white line LL to the right side in the lanewidth direction by a half (=W½) of the estimated/inferred lane width W1.In other words, the DSECU sets/determines, as the target traveling lineLd, a lane center line positioned at a midpoint between the recognizedleft white line LL and the extrapolated right white line LR.

B: When the DSECU can Recognize Only the Right White Line.

When the DSECU can recognize only the right white line LR on thetraveling lane in which the own vehicle is traveling, the DSECUextrapolates/determines, as a position of the left white line LL, aposition which is obtained through shifting a position of the recognizedright white line LR to the left side in the lane width direction by thedistance of the estimated/inferred lane width W1. The estimated/inferredlane width W1 is the latest value of the lane width W which wascalculated when both of the left white line LL and the right white lineLR were recognized. The latest value of the lane width has been storedin the RAM. The DSECU determines, as the target traveling line Ld, aline passing through positions, each of which is a position obtainedthrough shifting the recognized right white line LR to the left side inthe lane width direction by a half (=W½) of the estimated/inferred lanewidth W1. In other words, the DSECU sets/determines, as the targettraveling line Ld, a lane center line positioned at a midpoint betweenthe recognized right white line LR and the extrapolated left white lineLL.

<<A Case where the DSECU Cannot Recognize any of the Left White Line LLand the Right White Line LR (the No White Line Recognizable State)>>

When the DSECU can recognize neither the left white line LL nor theright white line LR, the DSECU cancels the lane keeping control. Thatis, in this case, the DSECU does not perform the lane keeping control.

<<An Upper Limit Guard Value of the Steering Angle and an Upper LimitGuard Value of the Steering Angle Speed>>

The DSECU limits the steering angle in such a manner that a magnitude ofthe steering angle does not become larger than the upper limit guardvalue of the steering angle while performing the lane keeping control.Hereinafter, this upper limit guard value of the steering angle is alsoreferred to as a “steering angle guard value”. In addition, the DSECUlimits the steering angle speed in such a manner that a magnitude of thesteering angle speed does not become larger than the upper limit guardvalue of the steering angle speed while performing the lane keepingcontrol, Hereinafter, this upper limit guard value of the steering anglespeed is also referred to as a “steering angle speed guard value”. As aresult, a behavior of the own vehicle is unlikely to abruptly/suddenlychange due to the change in the steering angle. Therefore, the ownvehicle can travel stably (a traveling stability of the own vehicle canbe ensured). The above descriptions are the outline of the lane keepingcontrol.

Next, the operation of the embodiment apparatus will be furtherdescribed with using an example shown in FIG. 4. In the example shown inFIG. 4, the own vehicle SV is traveling within a single lane in acertain road section in a direction shown by arrows. The DSECU isperforming the lane keeping control.

In a period from a time t0 to a time point immediately before a time t1,the DSECU recognizes both of the left white line LL and the right whiteline LR. Therefore, the DSECU sets the lane center line between the leftwhite line LL and a right white line LRa, as the target traveling lineLd1, based on “the recognized positions of the left line LL and theright line LR”. The DSECU performs the steering control in such a mannerthat the own vehicle SV travels along the target traveling line Ld1.

In a period from the time 1 to a time point immediately before a timet2, the DSECU can recognize only the left white line LL but cannotrecognize the right white line LR. In this case, the DSECUsets/determines the target traveling line Ld2 based on the position ofthe left white line LL and the estimated/inferred lane width W1 (thelane width W which was acquired at a time point immediately before thetime t1). That is, the DSECU sets/determines the lane center line as thetarget traveling line Ld2 on the premise that a right white line LRa ispresent at a position obtained through shifting a position of therecognized left white line LL to the right side by the distance of theestimated/inferred lane width W1. The DSECU performs the steeringcontrol in such a manner that the own vehicle SV travels along thetarget traveling line Ld2.

In a period from the time t2 to a time t3, the DSECU can recognize onlythe right white line LR, but cannot recognize the left white line LL. Inthis case, the DSECU sets/determines the target traveling line Ld3 basedon the position of the right white line LR and the estimated/inferredlane width W1 (the lane width W which was acquired at a time pointimmediately before the time t1). That is, the DSECU sets/determines thelane center line as the target traveling line Ld3 on the premise that aleft white line LLa is present at a position obtained through shifting aposition of the recognized right white line LR to the left side by thedistance of the estimated/inferred lane width W1. The DSECU performs thesteering control in such a manner that the own vehicle SV travels alongthe target traveling line Ld3.

As described above, in the period from the time t1 to the time t3, therecognition state of the white lines of the DSECU is the one side whiteline recognizable state. In the period from the time t1 to the time t2,the estimated/inferred lane width W1 is different from an actual/truelane width Wtrue. However, the position of the white line LRa on theunrecognized side (the right side) which the DSECU hasextrapolated/estimated does not deviate/differ from the “actual positionof the right white line LR” acquired in the period (period from the timet0 to the time point immediately before the time t1) during which therecognition state of the white lines of the DSECU was the both whitelines recognizable state. Further, the position of the left white linewhich the DSECU has recognized is the actual position of the left whiteline LL Therefore, the position of the target traveling line Ld2 doesnot deviate/differ greatly from the target traveling line Ld1 which hasbeen set by the DSECU while the recognition state of the white lines ofthe DSECU has been the both white line recognizable state.

However, at the time t2, a direction of a position of the recognizablewhite line with respect to the own vehicle changes from the leftdirection to the right direction (that is, the white line recognized bythe DSECU changes from the left white line to the right white line).Moreover, the estimated/inferred lane width W1 starts to differ from theactual/true lane width Wtrue at the time t2. As a result, the “positionof the white line LLa on the unrecognized side (the left side)”extrapolated/estimated by the DSECU deviates/differs from the actualposition of the left white line LL from the time t2. In addition, thisextrapolated/estimated position of the white line LLa on theunrecognized side (the left side) differs from the “position of the leftwhite line LL” which the DSECU has recognized up to that time (the timet2).

For the reasons described above, a position of the target traveling lineLd3 in the lane width direction deviates/differs from a position of thetarget traveling line Ld2 in the lane width direction. Therefore, sincethe target traveling line abruptly/suddenly changes before and aftertime t2, the steering state (or angle) of the own vehicle SVabruptly/suddenly changes especially when a change amount in theposition of the target traveling line in the lane width direction isrelatively large. This may cause a case to occur where the travelingstability of the own vehicle SV is degraded.

In view of the above, in the one side white line recognizable state, theDSECU of the embodiment apparatus decreases the upper limit guard valueof the steering angle (the steering angle guard value) and the upperlimit guard value of the steering angle speed (the steering angle speedguard value), when a specific recognition state occurs in which adirection of the position of the recognizable white line with respect tothe own vehicle changes. The specific recognition state occurs, when therecognizable white line (the lane which the DSECU can recognize) changesfrom only the left white line to only the right white line during theone side white line recognizable state, or when the recognizable whiteline changes from only the right white line to only the left white lineduring the one side white line recognizable state. That is, the specificrecognition state is a situation where the one recognizable lane markerhas changed between the left lane marker and the right lane marker.

Specifically, unless the specific recognition state occurs while thelane keeping control is being performed, the DSECU sets the steeringangle guard value to a predetermined first steering angle guard valueand sets the steering angle speed guard value to a predetermined firststeering angle speed guard value. Furthermore, when the specificrecognition state occurs while the lane keeping control is beingperformed, the DSECU sets the steering angle guard value to a “secondsteering angle guard value smaller than the first steering angle guardvalue” and sets the steering angle speed guard value to a “secondsteering angle speed guard value smaller than the first steering anglespeed guard value”. As a result, even if the specific recognition stateoccurs, the DSECU can reduce the possibility that a “situation where thetraveling stability of the own vehicle SV is degraded due to theabrupt/sudden change in the steering state (angle) of the own vehicleSV” occurs.

<Specific Operation>

Next, specific operations of the CPU (hereinafter, simply referred to asthe “CPU”) of the DSECU will be described. The CPU is configured toexecute a routine shown by a flowchart in FIG. 5, every time apredetermined time (Δt) elapses.

Therefore, when a predetermined timing arrives, the CPU startsprocessing from step 500 in the routine of FIG. 5 and proceeds to step505 to determine whether or not the lane keeping control is beingperformed. More specifically, the CPU starts the lane keeping controlwhen the CPU determines that a lane keeping control start conditionwhich will be described later is established. In addition, the CPUcontinues performing the lane keeping control until (the CPU determinesthat) a lane keeping control cancellation condition which will bedescribed is established. Therefore, the CPU determines that the lanekeeping control is being performed in a period from a time point whenthe lane keeping control start condition becomes established to a timepoint when the lane keeping control cancellation condition becomesestablished.

The lane keeping control start condition is established when all of thefollowing conditions (conditions 1 to 3) are established, for example.

The condition 1 is a condition established when performing the lanekeeping control is being selected through the operation of the operationswitch 18.

The condition 2 is a condition established when the vehicle speed SPD isequal to or greater/higher than a predetermined lower limit vehiclespeed and is equal to or smaller/lower than a predetermined upper limitvehicle speed.

The condition 3 is a condition established when at least any one of theleft white line LL and the right white line LR is being recognized bythe camera sensor 17 (and the DSECU).

The lane keeping control cancellation condition is established when anyone of the following conditions (conditions 4 to 6) is established, forexample.

The condition 4 is a condition established when performing the lanekeeping control is not being selected through the operation of theoperation switch 18.

The condition 5 is a condition established when the vehicle speed SPD islower than the lower limit vehicle speed.

The condition 6 is a condition established when the vehicle speed SPD ishigher than the upper limit vehicle speed.

When the lane keeping control is not being performed, the CPU makes a“No” determination at step 505 and then proceeds to step 595 totentatively terminate the present routine.

In contrast, when the lane keeping control is being performed, the CPUmakes a “Yes” determination at step 505 and then proceeds to step 510 toanalyze the image data transmitted from the camera sensor 17 to therebyattempt to recognize the left white line LL and the right white line LR.

Thereafter, the CPU proceeds to step 515 to determine whether or not anyof the left white line LL and the right white line has been unable to berecognized at the process of step 510 (i.e., none of the white lines isrecognized).

When neither the left white line LL nor the right white line has beenrecognized, the CPU makes a “Yes” determination and proceeds to step 520to cancel the lane keeping control. In this case, the CPU stopsperforming the lane keeping control. Thereafter, the CPU proceeds tostep 595 to tentatively terminate the present routine. It should benoted that the state where neither the left white line LL nor the rightwhite line LR can be recognized can be said to be one of cancellationconditions of the lane keeping control.

In contrast, when at least any one of the left white line LL and theright white line LR has been (able to be) recognized, the CPU makes a“No” determination at step 515 and proceeds to step 525 to determinewhether or not only one of the left white line LL and the right whiteline LR has been (able to be) recognized at step 510.

When both of the left white line LL and the right white line LR havebeen (able to be) recognized at step 510, the CPU makes a Nodetermination at step 525 and proceeds to step 526 to set/determine thetarget traveling line Ld based on the recognized left white line LL andthe recognized right white line LR in the manner described above. Inaddition, in this case, the CPU calculates a distance between therecognized left white line LL and the recognized right white line LRbased on the positions of those white lines to store the calculateddistance as the lane width W into the RAM. More specifically, the CPUsets the center line between the recognized left white line LL and therecognized right white line LR as the target traveling line Ld at step526. The target traveling line Ld set in this manner is very likely topass through the midpoint of the lane, and is very unlikely toabruptly/suddenly change/move in the lane width direction. Accordingly,a great/abrupt change in the steering state (angle) is unlikely tooccur. In other words, steering control reliability is high. For thisreason, the steering angle guard value and the steering angle speedguard value do not have to be decreased.

Therefore, in this case, the CPU proceeds to step 528 to set thesteering angle guard value θg to the first steering angle guard valueand set the steering angle speed guard value dθg to the first steeringangle speed guard value. It should be noted that when the steering angleguard value θg is already set to the first steering angle guard valueand the steering angle speed guard value dθg is set to the firststeering angle speed guard value at a time point at which the process ofthe step 528 is started, the CPU maintains the steering angle guardvalue θg and the steering angle speed guard value dθg at the firststeering angle guard value and the first steering angle speed guardvalue, respectively.

Thereafter, the CPU proceeds to step 555 to control the steering angleof the own vehicle SV in such a manner that the own vehicle SV travelsalong the target traveling line Ld set at step 526. More specifically,the CPU calculates the target steering angle θ* by using theabove-described Expression (1) or the above-described Expression (2).

Then, if a magnitude of the target steering angle θ* is equal to orlarger than the steering angle guard value θg (θg>0), the CPU changesthe target steering angle θ* in such a manner that the target steeringangle θ* has a magnitude equal to the steering angle guard value θg.That is, if the target steering angle θ* is a positive value and islarger than the steering angle guard value θg, the CPU sets the targetsteering angle θ* to “θg”. Furthermore, if the target steering angle θ*is a negative value and its magnitude |θ*| is larger than the steeringangle guard value θg, the CPU sets the target steering angle to “−θg”.

Furthermore, the CPU subtracts the “target steering angle θ* (that is,the previous target steering angle θ*p) calculated the predeterminedtime (Δt) before (in other words, calculated at a time point at whichthe present routine was previously executed)” from the “current targetsteering angle θ* (that is, the present target steering angle θ*n)calculated at the present time point”, to thereby calculate a targetsteering angle change amount (θ*n−θ*p). Furthermore, the CPU divides thetarget steering angle change amount (θ*n−θ*p) by the predetermined time(Δt) to thereby calculate a target steering angle change amount per unittime ((θ*n−θ*p)/Δt).

Then, when a magnitude of the target steering angle change amount perunit time |(θ*n−θ*p)/Δt| is equal to or larger than the steering anglespeed guard value dθg (dθg>0), the CPU changes θ*n in such a manner thatthe magnitude |(θ*n−θ*p)/Δt| is equal to the steering angle speed guardvalue dθg. That is, if the target steering angle change amount per unittime (θ*n−θ*p/Δt) is a positive value and is larger than the steeringangle speed guard value dθg, the CPU sets the target steering angle to a“value (=θ*p+dθg×Δt) obtained by adding the value dθg×Δt to the valueθ*p”. If the target steering change amount per unit time ((θ*n−θ*p)/Δt)is a negative value and its magnitude |(θ*n−θ*p)/Δt| is larger than thesteering angle speed guard value dθg, the CPU sets the target steeringangle to a “value (=θ*p−dθg×Δt) obtained by subtracting the value dθg×Δtfrom the value θ*p”.

The CPU controls the steering motor 62 in such a manner that the actualsteering angle θ coincides with the target steering angle θ* determinedin this manner. As a result, the steering angle θ is controlled in sucha manner that its magnitude |θ| does not exceed the steering angle guardvalue θg (this θg is currently set at the first steering angle guardvalue) and a magnitude |dθ/dt| of its change amount per unit time doesnot exceed the steering angle speed guard value dθg (this dθg iscurrently set at the first steering angle speed guard value).

In contrast, when only one of the left white line LL and the right whiteline LR has been (able to be) recognized at the time point at which theprocess of step 525 is executed, the CPU makes a “Yes” determination andproceeds to step 530 to estimate/infer the estimated/inferred lane widthW1 in the manner described above. That is, the CPU reads out the lanewidth W stored in the RAM as the estimated/inferred lane width W1.

Thereafter, the CPU proceeds to step 535 to set the target travelingline Ld in the manner described above, based on the position of the onerecognized white line and the estimated/inferred lane width W1 obtainedat step 530.

Subsequently, the CPU proceeds to step 540 to determine whether or not avalue of a guard value limiting flag Xd is “1”. The guard value limitingflag Xd is set to “1” through a guard value limiting determinationroutine shown in FIG. 6 described later, when the specific recognitionstate has occurred and thus a state of the own vehicle is in a state inwhich the steering angle guard value and the steering angle speed guardvalue should be decreased. When the state of the own vehicle is not insuch a state, the value of the guard value limiting flag Xd is set to“0” through the guard value limiting determination routine shown in FIG.6 described later.

When the guard value limiting flag Xd is not “1”, the CPU makes a “No”determination at step 540 and proceeds to step 528 to execute theprocess which has been already described at step 528. Furthermore, theCPU proceeds to step 555 to control the steering angle of the ownvehicle SV so as to have the own vehicle SV travel along the targettraveling line Ld which has been set at step 535. Thereafter, the CPUproceeds to step 595 to tentatively terminate the present routine.

In contrast, when the value of the guard limiting flag Xd is “1”, theCPU makes a “Yes” determination at step 540 and proceeds to step 550 todecrease the steering angle guard value θg and the steering angle speedguard value dθg. More specifically, the CPU sets the steering angleguard value θg to the second steering angle guard value and sets thesteering angle speed guard value dθg to the second steering angle speedguard value. It should be noted that the second steering angle guardvalue is smaller than the first steering angle guard value, and thesecond steering angle speed guard value is smaller than the firststeering angle speed guard value. Furthermore, when the steering angleguard value is already set to the second steering angle guard value andthe steering angle speed guard value is set to the second steering anglespeed guard value at a time point at which the process of the step 540is started, the CPU maintains the steering angle guard value and thesteering angle speed guard value at the second steering angle guardvalue and the second steering angle speed guard value, respectively.

Thereafter, the CPU proceeds to step 555 to control the steering angleof the own vehicle SV in such a manner that the own vehicle SV travelsalong the target traveling line Ld set at step 535. As a result, thesteering angle θ is controlled in such a manner that its magnitude |θ|does not exceed the steering angle guard value θg (this θg is currentlyset at the second steering angle guard value) and the magnitude |dθ/dt|of its change amount per unit time does not exceed the steering anglespeed guard value dθg (this dθg is currently set at the second steeringangle speed guard value). Then, the CPU proceeds to step 595 totentatively terminate the present routine.

Next, the guard value limiting determination routine will be described,referring to FIG. 6. The CPU is configured to execute a routine shown bya flowchart of FIG. 6, every time the predetermined time elapses.

Therefore, when a predetermined timing arrives, the CPU startsprocessing from step 600 of the routine of FIG. 6 and proceeds to step605 to determine whether or not the lane keeping control is beingperformed, similarly to step 505.

When the lane keeping control is being performed, the CPU makes a “Yes”determination at step 605 and proceeds to step 610 to determine whetheror not the value of the guard value limiting flag Xd is “0”, It shouldbe noted the value of the guard value limiting flag Xd is set to “0”through an initialization routine executed by the CPU when an ignitionswitch (not shown) provided for the own vehicle SV is changed from an ONposition to an OFF position.

When the guard value limiting flag Xd is “0”, the CPU makes a “Yes”determination at step 610 to proceed to step 615 to determine whether ornot the following determination condition (hereinafter, referred to as a“determination condition 1”) of the specific recognition stateoccurrence is established.

The determination condition 1 is a condition established when both ofthe following conditions are satisfied.

-   -   only one of the left white line LL and the right white line LR        (either the line LL or the line LR) was recognized at the time        point at which the process of step 510 was executed when the        present routine was previously executed the predetermined time        before the present time (that is, in the previous calculation);        and    -   a direction of the white line recognized with respect to the own        vehicle SV in the previous calculation and a direction of the        white line recognized with respect to the own vehicle SV in the        current calculation (execution) of the present routine (in the        present calculation) are difference from each other (that is,        the direction of the position of the white line recognized at        the previous calculation differs from the direction of the        position of the white line recognized at the present        calculation).

That is, the determination condition 1 is satisfied,

-   -   when each of a recognition states of the lane markers (white        lines) at the present time point and at the time point the        predetermined time before the present time point is the one side        lane marker recognizable state; and    -   when the one recognized lane marker (the lane marker which has        been recognized) changes from the left lane marker to the right        lane marker or vice versa in a period between the present time        point and the time point the predetermined time before the        present time point.

When the determination condition 1 is established, it can be determinedthat the specific recognition state has occurred. In this case, theposition of the target traveling line may greatly deviate/differ fromthe position at a time point immediately before the specific recognitionstate has occurred. Thus, steering state (angle) of the own vehicle SVmay abruptly/suddenly change greatly. For this reason, it is preferable(and/or required) to decrease the steering angle guard value and thesteering angle speed guard value. Therefore, when the determinationcondition 1 is established, the CPU makes a “Yes” determination at step615 and proceeds to step 620 to set the value of the guard valuelimiting flag Xd to “1”.

Thereafter, the CPU proceeds to step 630 to determine whether or not theown vehicle has crossed the white line (i.e., recognized white line)which has been recognized. For example, it is determined that thedetermination condition 1 is established at step 615, and thus, it isdetermined that the specific recognition state has occurred, under asituation where only a right white line has been recognized, when theown vehicle has crossed the right white line, and then, a case hasoccurred where only that right white line (which becomes a left whiteline after the own vehicle has crossed that right white line) can berecognized.

However, in this case, the own vehicle has already changed lanes.Therefore, the target traveling line is changed from the original targettraveling line to a lane center line of a lane in which the own vehicleis travelling after the own vehicle has changed lanes. In this case, itis preferable to quickly change the steering angle to have the ownvehicle approach the new target traveling line after the lane change.For this reason, it is not necessary to decrease the steering angleguard value and the steering angle speed guard value in this case.

Therefore, when the own vehicle SV has already crossed the “recognizedwhite line”, the CPU makes a “Yes” determination to proceed to step 640to set the value of the guard value limiting flag Xd to “0”. Thereafter,the CPU proceeds to step 695 to tentatively terminate the presentroutine. As a result, the value of the guard value limiting flag Xd ismaintained at “0”.

In contrast, when the own vehicle SV has not crossed the “recognizedwhite line” yet, the CPU makes a “No” determination and proceeds to step695 to tentatively terminate the present routine.

In contrast, when the determination condition 1 is not established atthe time point at which the CPU execute the process of step 615, it canbe determined that the specific recognition state has not occurred yet.In this case, the possibility that the target traveling line Ld changesabruptly/suddenly is very low, and thus, an abrupt/sudden change insteering state (angle) is very unlikely to occur (in other words, thereliability of steering control is high). For this reason, each of thesteering angle guard value and the steering angle speed guard value doesnot have to be set to a small value. Therefore, when the determinationcondition 1 is not established, the CPU makes a “No” determination atstep 615 and proceeds directly to step 695 to tentatively terminate thepresent routine. As a result, the value of the guard value limiting flagXd is maintained at “0”.

Furthermore, when the value of the guard value is “1” at the time pointat which the CPU execute the process of step 610, the CPU makes a “No”determination at step 610 and proceeds to step 635 to determine whetheror not a cancellation condition is established. The cancellationcondition is a condition established when any one of the followingconditions (“the condition A and the condition B”) is established.

The condition A: the condition A is a condition established when the CPUcan recognize both the left white line LL and the right white line LR.

The condition B: the condition B is a condition established when aparticular state has continued for a predetermined time period from atime point at which the specific recognition state occurred. Theparticular state is a state where the one side lane marker recognizablestate is occurring and the specific recognition state does not occur.

It should be noted that the CPU may determine whether or not theabove-described condition B is established in accordance with at leastany one of the following method 1 and the following method 2.

The method 1: the CPU measures/calculates a duration time of theparticular state from a time point at which the specific recognitionstate occurred. The CPU determines that the above-described condition Bis established, when the measured duration time becomes equal to orlonger than a threshold duration,

The method 2: the CPU measures/calculates a traveling distance of theown vehicle SV under the above particular state from the time point atwhich the specific recognition state occurred. The CPU determines thatthe above-described condition B is established, when the measuredtraveling distance becomes equal to or longer than a threshold travelingdistance.

When the cancellation condition is established, it can be determinedthat a steering state (angle) of the own vehicle becomes unlikely toabruptly/suddenly change after the specific recognition state occurred.Therefore, when the cancellation condition is established, the CPU makesa “Yes” determination at step 635 and proceeds to step 640 to set thevalue of the guard value limiting flag Xd to “0”. Thereafter, the CPUproceeds to step 695 to tentatively terminate the present routine.

In contrast, when the cancellation condition is not established, the CPUmakes a “No” determination at step 635 and proceeds directly to step 695to terminate the present routine. As a result, the value of the guardvalue limiting flag Xd is maintained at “1”.

It should be noted that when the lane keeping control is not beingperformed at a time point at which the process of the step 605 isexecuted, the CPU makes a “No” determination at step 605 to proceed tostep 645, at which the CPU set the value of the guard limiting flag Xdto “0”. Thereafter, the CPU proceeds to step 695 to tentativelyterminate the present routine.

According to the above-described embodiment apparatus, the followingeffects are obtained. The embodiment apparatus is capable of reducingthe possibility that the steering state (angle) of the own vehicle SVabruptly/suddenly changes when the specific recognition state occurswhere the direction of the one recognizable white line with respect tothe own vehicle SV changes from the left to the right or vice versawhile only one of the left white line and the right white line has beenable to be recognized. As a result, the embodiment apparatus can reducethe possibility that the traveling stability of the own vehicle isdegraded.

Modified Examples

Although the embodiment of the present invention has been specificallydescribed above, the present invention is not limited to the aboveembodiment, and various modified examples based on the technical ideawithin the scope of the present invention can be adopted.

For example, although the embodiment apparatus detects the white linesby using the camera sensor 17, a method of detecting the white lines isnot limited to this. For example, the modified example of the embodimentis provided with radar sensors (not shown) (e.g., laser radars) todetect the white lines using any one of the radar sensors.

One of the modified examples of the embodiment may estimate/infer theestimated/inferred lane width W1 according to a method different fromthe above-described method to thereby acquire the width W1. For example,one of the modified examples may estimate/infer the estimated/inferredlane width W1 based on the road information acquired from the mapdatabase 92 by the navigation ECU 90. Alternatively, for example, one ofthe modified examples may estimate/infer the estimated/inferred lanewidth W1 based on the road information acquired through performing thewireless communication with the roadside communication device installedon the road using the wireless communication device 101.

For example, one of the modified examples may be configured to limit thesteering angle using the steering angle guard value without limiting thesteering angle (the steering angle speed) using the steering angle speedguard value. Alternatively, one of the modified examples may beconfigured to limit the steering angle (the steering angle speed) usingthe steering angle speed guard value without limiting the steering angleusing the steering angle guard value.

For example, one of the modified examples may set the steering angleguard value and the steering angle speed guard value to the followingvalues depending on each of the lane marker recognition states of theDSECU.

-   -   When the lane marker recognition state of the DSECU is the “both        lane markers recognizable state” and the value of the guard        value limiting flag Xd is “0”, the steering angle guard value is        set to the first steering angle guard value, and the steering        angle speed value is set to the first steering angle speed guard        value.

This is the same as the operation of the above-described embodimentapparatus,

-   -   When the lane marker recognition state of the DSECU is the “one        side lane marker recognizable state” and the value of the guard        value limiting flag Xd is “1”, the steering angle guard value is        set to the second steering angle guard value, and the steering        angle speed value is set to the second steering angle speed        guard value.

This is also the same as the operation of the above-described embodimentapparatus.

-   -   When the lane marker recognition state of the DSECU is the “one        side lane marker recognizable state” and the value of the guard        value limiting flag Xd is “0”, the steering angle guard value is        set to a third steering angle guard value, and the steering        angle speed value is set to a third steering angle speed guard        value.

In the above case, it should be noted that the first steering angleguard value is equal to or larger than the third steering angle guardvalue, and the third steering angle guard value is larger than thesecond steering angle guard value (that is, the first steering angleguard value≥the third steering angle guard value>the second steeringangle guard value). Similarly, the first steering angle speed guardvalue is equal to or larger than the third steering angle speed guardvalue, and the third steering angle speed guard value is larger than thesecond steering angle speed guard value (that is, the first steeringangle speed guard value≥the third steering angle speed guard value>thesecond steering angle speed guard value).

For example, one of the modified examples may decrease only one of thesteering angle guard value and the steering angle guard value at step550 in FIG. 5.

For example, the target traveling line is not necessarily be the lanecenter line, as long as the target traveling line may be a linedetermined based on the left white line LL and the right white line LR.For example, the DSECU may set, as the target traveling line Ld, a linepassing through positions apart from the “lane center line passingthrough midpoints between the left white line LL and the right whiteline LR” by a predetermined distance in the lane width direction. Such amethod of setting the target traveling line Ld can be used regardless ofwhether the lane marker recognition state is the “both lane markersrecognizable state” or the “one side lane marker recognizable state”.

What is claimed is:
 1. A vehicle traveling support apparatus comprising:lane marker recognition means for recognizing a lane marker on atraveling lane in which an own vehicle is traveling; control means forperforming a lane keeping control to control a steering angle of saidown vehicle so as to have said own vehicle travel along a targettraveling line set based on at least said lane marker; and estimatinglane width means for estimating a lane width of said traveling lane,wherein, said control means is configured: when a recognition state ofsaid lane marker by said lane marker recognition means is a both lanemarkers recognizable state which is a state where said lane markerrecognition means can recognize both a left lane marker on a left sidewith respect to said own vehicle and a right lane marker on a right sidewith respect to said own vehicle, to set said target traveling linebased on said left lane marker and said right lane marker; and toperform a first limiting process including one of, a first steeringangle magnitude limiting process for limiting said steering angle insuch a manner that a magnitude of said steering angle does not exceed afirst steering angle guard value, and a first steering angle speedmagnitude limiting process for limiting said steering angle in such amanner that a magnitude of a steering angle speed which is an amount ofchange per unit time of said steering angle does not exceed a firststeering angle speed guard value, and wherein, said control means isconfigured: to set said target traveling line based on said lane markerwhich is recognized by said lane marker recognition means and saidestimated lane width, when said recognition state of said lane marker bysaid lane marker recognition means is a one side lane markerrecognizable state which is a state where said lane marker recognitionmeans can recognize only one of said left lane marker and said rightlane marker; and to perform, when a specific recognition state occurswhere said lane marker recognized by said lane marker recognition meanschanges from said left lane marker to said right lane marker or viceversa under said one side lane marker recognizable state, a secondlimiting process including one of, a second steering angle magnitudelimiting process, in place of said first steering angle magnitudelimiting process, for limiting said steering angle in such a manner thatsaid magnitude of said steering angle does not exceed a second steeringangle guard value smaller than said first steering angle guard value,and a second steering angle speed magnitude limiting process, in placeof said first steering angle speed magnitude limiting process, forlimiting said steering angle in such a manner that said magnitude ofsaid steering angle does not exceed a second steering angle speed guardvalue smaller than said first steering angle speed guard value.
 2. Thevehicle traveling support apparatus according to claim 1, wherein, saidcontrol means is configured to perform said first limiting process inplace of said second limiting process when said specific recognitionstate has occurred after said own vehicle crossed said lane markerrecognized by said lane marker recognition means, while said recognitionstate of said lane marker by said lane markers recognition means is saidone side lane marker recognizable state.
 3. The vehicle travelingsupport apparatus according to claim 1, wherein, said control means isconfigured, when a particular state has continued for a predetermineperiod from a time point at which said specific recognition stateoccurred, said particular state being a state where said recognitionstate of said lane marker by said lane marker recognition means is saidone side lane marker recognizable state and said particular recognitionstate does not occur, to perform a third limiting process including atleast any one of, a third steering angle magnitude limiting process, inplace of said second steering angle magnitude limiting process, for saidsteering angle in such a manner that said magnitude of said steeringangle does not exceed a third steering angle guard value larger thansaid second steering angle guard value and equal to or smaller than saidfirst steering angle guard value; and a third steering angle speedmagnitude limiting process, in place of said third steering angle speedmagnitude limiting process, for limiting said steering angle speed insuch a manner that said magnitude of said steering angle speed does notexceed a third steering angle speed guard value larger than said secondsteering angle speed guard value and equal to or smaller than said firststeering angle speed guard value.
 4. The vehicle traveling supportapparatus according to claim 3, wherein, said control means isconfigured to determine that said particular state has continued forsaid predetermine period, when a duration time of said particular statefrom a time point at which said specific recognition state occurredbecomes equal to or longer than a threshold duration time.
 5. Thevehicle traveling support apparatus according to claim 3, wherein, saidcontrol means is configured to determine that said particular state hascontinued for said predetermine period, when a traveling distance ofsaid own vehicle under said particular state from a time point at whichsaid specific recognition state occurred becomes equal to or longer thana threshold traveling distance.